1. Field of the Invention
This invention relates to a process for manufacturing semiconductor devices and more particularly to an improved step-and-repeat method of exposing arrays of patterns in the formation of components of integrated circuit semiconductor devices.
2. Description of Related Art
The prior art deals simply with breaking up patterns and putting them on separate masks which is a problem since the reticle mask must be removed from the stepper and the second reticle mask must be inserted to make an exposure.
Examples of multiple exposure processes are shown by Kemp U.S. Pat. No. 5,308,741 and Neisser U.S. Pat. No. 5,563,012. The Neisser patent deals with taking an array and subdividing it into separate overlay masks, followed by forming the final pattern by several exposures using the overlay masks.
U.S. Pat. No. 5,563,012 of Neisser for xe2x80x9cMulti Mask Method for Selective Mask Feature Enhancementxe2x80x9d describes a method for forming a mask by dividing or grouping pattern shapes into shape groups according to feature types. Two or more overlay masks can contain complementary pattern shapes which are employed sequentially. As stated at Col. 3, lines 46-48, xe2x80x9cThus the entire pattern of the original mask is printed by successive exposure of the overlay mask patterns on to the photoresist.xe2x80x9d This requires removing a mask and substituting one or more complementary masks for the original mask. A key difference is that the pattern itself is not altered or overlapped, as in this disclosure.
U.S. Pat. No. 5,308,741 of Kemp for xe2x80x9cLithographic Method Using Double Exposure Techniques, Mask Position Shifting and Light Phase Shiftingxe2x80x9d describes a method of xe2x80x9cusing double exposures, physical mask shifting and light phase shifting to form masking features on a substrate masking layer. A first Phase Shifting Mask (PSM) is used to form at least one image by exposing a mask at a first location. A second exposure is made using either a second PSM or reusing the first PSM. The PSM used in the second exposure is positioned to partially overlap the original position of the first PSM. If the first PSM is reused, it is shifted to a second exposure position. In either case, in the second exposure the regions of the substrate with first and second exposures have some common unexposed regions which are used to form the masking features. If the first mask is reused it can be shifted by a rectilinear or rotational displacement into the second position. After removal of the exposed resist there are a plurality of unexposed regions on the substrate.
U.S. Pat. No. 5,815,245 of Thomas et al. for xe2x80x9cScanning Lithography System with Opposing Motionxe2x80x9d describes a scanning photolithography system which uses opposing motion of a reticle and a blank to compensate for image reversal by a projection system such as a conventional Wynne-Dyson optical system which forms a reverted image on a blank. During scanning the reticle moves in a direction opposite to the direction in which the blank moves. Linear motors are used. See Col. 14, lines 16-20, where it is stated xe2x80x9cTypically, relative motion is provided by a microstepper or other precision motor driving the secondary stage under control of a conventional control mechanism receiving feedback from the reticle alignment system.xe2x80x9d
One of the most severe problems for shrinking down existing designs is the impact of line shortening and the problem is exacerbated for smaller ground rules, because of the effects of resist diffusion, comer rounding, mask making, etc.
Dense and small patterns suffer from the problem of line shortening and pattern fidelity. This makes it difficult or impossible to maintain the shape of the pattern. This invention shows novel polygonally-shaped pattern layouts, based on nano-stepping, that not only reduce line shortening but also improve pattern fidelity to such a degree that it exceeds the pattern fidelity obtainable with single exposure techniques.
In accordance with this invention, a method for exposing a workpiece in a dual exposure step-and-repeat process starts by forming a design for a reticle mask. Deconstruct the design for the reticle mask by removing a set(s) of the features that are juxtaposed to form hollow polygonally-shaped clusters with a gap in the center. Form unexposed resist on the workpiece. Load the workpiece and the reticle mask into the stepper. Expose the workpiece through the reticle mask. Reposition the workpiece by a nanostep. Then expose the workpiece through the reticle mask after the repositioning. Test whether the plural exposure process is finished. If the result of the test is NO the process loops back to repeat some of the above steps. Otherwise the process has been completed. An overlay mark is produced by plural exposures of a single mark. A dead zone is provided surrounding an array region in which printing occurs subsequent to exposure in an original exposure. Alternatively, the workpiece can be fully exposed first by stepping a series of full steps, then going back to the starting position, making a nanostep to reset the starting position and re-exposing from the reset starting position in the same way with full steps from the nanostepped position. The clusters may be in the shape of a hexagon or a diamond.